Clinical chemistry analyzers, such as those manufactured by Abbott Laboratories and the Johnson and Johnson Company, among others, having reagent supplies on board for use in preparing wet immunoassays are sensitive to the effect of reagent evaporation, thereby causing prediction shifts with storage time. The change in the concentration of reagents that occurs with evaporation causes the reaction concentrations in a reaction vessel, such as a cuvette, to change, thereby causing the prediction shifts. Assay systems with a large reagent concentration sensitivity, such as EMIT TDM assays, for example, need effective evaporation control to achieve acceptable onboard stability times. Stability goals require that reagents be stable for 2 or more weeks onboard a clinical analyzer.
Most known “wet” analyzer systems use one of three methods to control such evaporation effects, as found with reagent supplies containing a plurality of bottles of at least one reagent. The first method is to package large volume reagent bottles on board and to use relatively large amounts of fluid in each test. This method does not reduce evaporation, but the effect is spread across a large fluid volume so that the effect on reagent concentration is minimized. This method of control requires a large reagent storage area within the analyzer and increases the cost per test as more fluid is used in each test.
A second method of evaporation control is to cap the reagent bottles between each use. This method does reduce evaporation effects, but only while the bottles are capped, and further requires a cap opening mechanism which must be disposed within the analyzer to open and close the caps between uses. This mechanism and its associated control adds significantly to the overall cost and complexity of the analyzer. In addition, evaporation still occurs as each bottle opening operation tends to flush out air contained in the bottle and in fact, actually increases or promotes evaporation. A more expensive bottle design is also needed using this method, as the bottle caps that are used during long-term storage must be compatible with the bottle opening mechanism.
A third known evaporation control method is to restrict the size of the metering access hole in the top of the reagent supply bottle. The bottles are stored uncapped within the reagent supply and using a small diameter hole reduces evaporation out of the reagent supply. This form of control works well with syringe metering systems that utilize relatively narrow metal tubes (˜2 mm in diameter), but a metering system using disposable metering tips having larger diameters on the order of about 10 mm or more requires a reagent bottle(s) having a relatively large accommodating metering access hole or opening which does not control evaporation acceptably. In addition, the diameter of the necks of the larger diameter reagent bottles must also be large enough to accept a relatively large metering tip and associated mechanism which passes into the bottle interior in order to aspirate fluid therefrom.